Manufacture of pulp from colored rags



FIP8399 Patented Oct. 3, 1933 UNITED STATES PATENT OFFICE MANUFACTURE OF PULP FROM COLORED RAGS Samuel Lenher, Wilmington, Del., and Harry Fletcher Lewis, Appleton, Wis., assignors to No Drawing. Application October 12, 1932 Serial No. 637,548

11 Claims. (01. 92-14) This invention relates to the manufacture of paper pulp and. more particularly to an improved process for making a high grade pulp from colored rags.

The'various cooking processes used in the past for the utilization of colored rags for high grade white paper stock have not been technically successful because the processes failed to yield a pulp which met all of the specifications set up for a white rag paper half-stock. Cooking processes for colored rags used in the past which have yielded a light colored or white stock have been weak owing to the excessive cooking time or the high chemicals concentration employed to remove the coloring matter. Cooking processes yielding strong stocks have not efiected a satisfactory color removal and the half stock has, of necessity, been graded down or blended with a white stock.

It is the present practice in the rag paper trade to cook colored rags with relatively large amounts of alkali for long periods of time to effect a color removal which enables the stock to be bleached to a light color. These cooking conditions are so drastic that the stock which was originally strong comes from the cock as a greatly degraded half-stock. These single stage cooking operations are made with the purpose of cleansing and opening up the rags and removing or destroying the color in one operation. One of the most desirable sources of rag cellulose is in the new blue denim clippings (new blue overall stock). The denim fibres have all the strength of the original fibres as the denims are made from unbleached cotton, however, the usual alkali cooking treatment necessary to effect a satisfactory color removal of the dyestuif (indigo) very greatly degrades the cellulose yielding a weak half-stock.

An object of this invention is a process for the preparation of high grade rag paper stock from colored rags. A further object is to make available for the manufacture of high grade rag paper classes of colored rags which it has not been possible to use for other than colored or lower grade papers heretofore. A still further object is to effect a thorough color removal from dyed rags without destroying or greatly diminishing their original strength. Other objects will appear hereinafter.

These objects are accomplished by the following invention which comprises the cooking of colored rags in closed vessels in a multiple stage process, the first step of which involves the use of caustic alkalies in solution, and the succeeding steps of which involve the use of wash water and of hydrosulfite or hydrosulfite containing compounds in solution, with or without the use of additional alkali. The process includes washing the rag stock in the cooking vessel with water between the various cooking steps.

The present process separates the cooking process into two distinct steps, both of which are carried out in the same equipment. The first step is termed a pre-cook and consists in the saponification and solubilization of fats, greases, and dirt in and on the fibres by the use of relatively small amounts of alkali in solution for short periods of time. This pre-cook comprises cooking the rags in a closed rotary boiler with a caustic alkali, such as lime and soda ash, or sodium hydroxide. The second step, after the precooking liquor has been blown off, comprises treating the rags for short periods of time, as for instance from 15 minutes to one hour, with sodium hydrosulfite or a hydrosulfite containing compound at a temperature between C. and 105 C. in order to remove or discharge the color from the rags. The hydrosulfite is preferably added in successive amounts as is indicated by the required color removal, or may be added in a single batch which is suflicient to remove the color. In general, it is most efficient and economical to add the hydrosulfite in successive small amounts, and this is the preferred method of color removal in our process.

Our improved rag cooking process requires no special equipment and can be used in the usual rotary boilers employed at present in the manufacture of rag paper with very minor changes. The rotary boilers should be equipped with a large blow-01f outlet for the rapid removal of the cooking and washing liquors. The boilers should also be equipped with water connections at two or more places widely spaced along the rotary or with a distributor pipe along the inside of the rotary fitted with holes or vents which will distribute the cooking, stripping and washing liquors thoroughly throughout the length of the rotary. The rotary should be turned continually during the cooking operations in this process.

In developing our process we have found that the alkaline cook, when carried out only to cleanse the rags and not to effect color removal requires a very much lower alkalinity than in the current cooking practice. Our experience has shown that there are a great number of pre-cooks which can be used in our process, and we do not wish our process to be limited to the examples given to illustrate our process. The alkalies which we prefer to use are lime, either burnt or slaked lime, soda ash, and caustic soda. The lime and caustic soda may be used alone or with soda ash; in general, our preferred procedure is to use lime and soda ash together, and to use caustic soda alone. The kind of alkali which is used, and the actual amounts, will depend on the class of rag stock used, on the condition of the rags, and on the desired color and strength. of the half-stock. It is obvious to those skilled in the art that the alkaline pre-cook is subject to a search iitfl great number of variations and that only limits of desired conditions can be outlined. We have found that the time of a pre-cook, the steam pressure (i. e., temperature), and the amount of alkali used are closely related, 1. e., with large amounts of alkali cooking times of an hour suffice, while with small amounts of alkali cooking times of three to four hours are necessary to properly cleanse and open up the rags. In our process we have found that four hours is the longest time which is required in a pre-cook; the shortest practicable time has been found to be one hour, although still shorter times can be used. We have found that in the pre-cook it is desirable to use from 3% to 6% of lime and 1% to 2% of soda ash (based on the weight of the rags), or 6% to 10% of lime alone, or from 2% to 6% .of caustic soda alone. Examples of pre-cooks which we have carried out on different grades of rags and which yielded a high grade half-stock are as follows:

EXAMPLE I-PART I Pre-cook on new blue overall stock 6% lime (CaO).

3% soda ash.

Ratio of rags to cooking liquor, 1: 2.5.

Cooking time 3 hours at 40 lbs. steam pressure. Viscosity of half-stock 203.

EXAMPLE IIPAR'1 I Pre-coolc on new blue overall stock 3.5% caustic soda (NaOH).

Ratio of rags to cooking liquor, 1: 2.5.

Cooking time 3.5 hours at 40 lbs. steam pressure. Viscosity of half-stock 250.

EXAMPLE III-PART I Pre-coolc on new blue overall stock 3.5% caustic soda.

Ratio of rags to cooking liquor, 1125.

Cooking time 1 hour at lbs. steam pressure. Viscosity of half-stock 217.

EXAMPLE IVPART I Pre-cook on thirds and blues 3.5% caustic soda.

Ratio of rags to cooking liquor, 122.5.

Cooking time 1.5 hours at 60 lbs. steam pressure. Viscosity of cooked half-stock 57.0.

Viscosity of uncooked stock 65.5.

After treating the rags according to any of the pre-cooks outlined above, the final cook is given to remove the color by stripping with bydrosulfite or hydrosulfite containing liquors. The pre-cooking liquor is blown from the rotary and the rags washed with warm or cold water. The wash water is drained from the boiler and the stripping liquor added to the rags in the rotary in such a way that air is not introduced. The stripping is most easily accomplished by repeated extractions with an alkaline solution containing 0.5 to 1.0% (on the dry weight of the rags) of sodium hydrosulfite or its reducing equivalent of hydrosulfite containing liquor. The rags are cooked with alkaline hydrosulfite for from one hour to fifteen minutes at from C. to 105C.while the rotary is turned. Concentrations of hydrosulfite greater than 1 are uneconomical, and while under certain conditions, concentrations as low as 0.25% can be used with success, it is not recommended in our process because of the greatly increased time of operation. Usually three extractions with 0.5% hydrosulfite are necessary to remove the greater part of the dyestuffs, but with careful operation two extractions with 0.5% hydrosulfite have been found to be highly satisfactory. The hydrosulfite is applied to the rags immediately after the pre-cook (or Wash, if the wash is used after the pre-cook). The hydrosulfite can be applied in dry form or as a solution or as a liquor containing hydrosulfite compound. If air is excluded from the digester during the admission of the hydrosulfite, oxidation of the hydrosulfite is prevented. Admission, of the hydrosulfite in a solution through an inlet in the bottom of the rotary to flood the rags is advantageous. The hydrosulfite solution should be alkaline at all times, and it is essential to the successful operation of this process that the hydrosulfite stripping liquors be maintained alkaline. Addition of 0.25% caustic soda with the hydrosulfite will insure an alkaline stripping condition.

The following examples of hydrosulfite cooks following alkaline pre-cooks illustrate this process. These examples of hydrosulfite cooks complete the pre-cooks given under the corresponding numbers above.

EXAMPLE I-PART II Wash at 100 C. after pre-cook.

Four 0.5% sodium hydrosulfite extractions at Half-stock bleached to a high white.

Viscosity of bleached stock 103.

EXAMPLE II-PART II Wash at C. after pre-cook.

Two 0.5% sodium hydrosulfite extractions at Half-stock bleached to a white with yellow tint.

Viscosity of bleached stock 78.

EXAMPLE IIIPART II Wash at 100 C. after pre-cook.

Two 0.5% sodium hydrosulfite extractions at Half-stock bleached to a clear white.

Viscosity of bleached stock 153.

EXAMPLE IV--PARr II Wash at 100 C. after pre-cook.

One 0.5% sodium hydrosulfite extraction at Half-stock bleached to a clear white.

Viscosity of bleached stock 52.1 (original viscosity of uncooked stock 65.5).

By the term hydrosulfite as used herein we mean an alkali metal salt containing the bivalent radical S204, or an addition compound of such salt with formaldehyde, HCHO, and water of hydration. These compounds consist of the sulfoxylates of the alkali metals and include hydrosulfites and sulfoxylate-formaldehyde addition compounds. It is to be understood that among the suitable agents there is included mixtures of alkali metal bisulfites, such as sodium, potassium, or calcium bisulfites, with zinc dust, which react in solution to yield hydrosulfites.

This invention permits of a number of variations. The principal variables are: the class of rags, alkali concentration, hydrosulfite concentration, kind of alkali used, time of cooking, steam pressure (i. e., temperature of cooking), and ratio of weight of rags to weight of cooking liquor. This process can be used with all classes of rags, especially of colored rags, except rags re iBEiFi LIBERAUQ was.

which are dyed ainly with vat dyestufis. The Viscosity of highly ble h d stock 7 alkali concentration depends, as we have outlined above, on cooking time and steam pressure, and on the kind of alkali; with lime, concentrations of less than 10% of lime (CaO) based on the dry weight of the rags is preferred; with lime and soda ash, less than 6% lime and 3% soda ash are preferred; with caustic soda, less than 3.5% is preferred. The hydrosulfite concentration is preferably less than 0.5% of the dry weight of the rags at a single addition; it is preferred that not more than three additions are made. The cooking time is dependent on the alkali concentration and the steam pressure as well as on the class of the rags; the preferred cooking time is less than four hours and not less than one hour. Steam pressure should not be less than 15 lbs. with high alkali cooks or for long cooking times (four hours and should not be more than 70 lbs. for low a kali cooks for short periods of time (one hour). The ratio of weight of rags to weight of cooking liquor can be varied from 1: 1 to 1: 4, but the preferred ratio is from 1: 2 to 1: 3.

This process is applicable to cooking of and removal of color from rags dyed with indigo, azo and sulfur colors in the classes of direct, acid and basic, and sulfur dyestuffs. This process is not applicable to the removal of color from fibers dyed with vat dyestuffs other than indigo. It is to be understood, therefore, that the reference to colored rags in the claims means rags dyed with indigo and with dyestuffs of any class except the anthraquinone dyes and vat dyestuffs.

Our process is useful for the preparation of a strong, light-colored pulp from colored rag stock. It is especially useful for the preparation of high grade half-stock for the manufacture of fine rag paper.

By means of our improved process, it is possible to prepare from highly colored rags a pulp which has a light color, good bleachability and great strength. The present method enables the rag paper manufacturer to produce a high grade pulp from a cheap class of rag stock at a low process cost.

The advantages of the present process with respect to avoidance of degradation in the alkali cook will be apparent by the following comparison resulting from tests made on stock prepared by the current alkali cooking methods and by our new process. The original viscosity (i. e., viscosity of a cuprammonium solution of the rags measured under standard conditions, which is accepted as a test of the strength of paper stock) was 370; the viscosity after cooking in the usual manner was 123; and after bleaching the viscosity fell to below 10, which is lower than the viscosity of a bleached stock from old rags. The hydrosulfite multiple stage cook of the present invention gives the following results with the same stock: original viscosity 3'70, viscosity after cooking 217, viscosity after bleaching to a high white The following example of a mill run on new blue overall stock is given as a comparison of the results of our process with the usual treatment of the same stock as currently practiced:

10% lime, 3% soda ash.

Cooking time 15 hours at 35 lbs. steam pressure. Ratio of rags to cooking liquor, 1: 2.5.

No hydrosulfite used.

Viscosity of half-stock 82.5.

Color of highly bleached (2% chlorine) stock remains a distinct blue.

The stock thus prepared was very weak, in comparison to the product obtained in accordance with the examples illustrating the present invention, had a poor color, and is representative of the conventional cooking practice.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the following claims:

We claim:

1. A process for the manufacture of paper pulp from colored rags which comprises treating the rags with a hot solution of causticalkali, removing the alkaline liquor fror'rf'the treated rags, washing the rags from which the liquor has been removed with water, treating the washed rags with an alkaline hydrosulfite compound, removing the residual hydrosulfite'liduor from the rags, and again washing the rags with water.

2. The process set forth in claim 1 in which the caustic alkali is lime and soda ash.

3. The process set forth in claim 1 in which the caustic alkali is caustic soda.

4. The process set forth in claim 1 in which the hydrosulfite is sodium hydrosulfite.

5. The process set forth in claim 1 in which the treatment with caustic alkali is conducted under a pressure from about 15 to 70 lbs. per square inch with a mixture of from 3% to 6% lime and 1% to 2% soda ash.

6. The process set forth in claim 1 in which the treatment with caustic alkali is conducted under a pressure of 15 to '70 lbs. per square inch with 2% to 6% caustic soda.

7. The process set forth in claim 1 in which the hydrosulfite concentration is not less than 0.25%.

8. The process set forth in claim 1 in which the hydrosulfite concentration is from 0.5% to 1.0%.

9. The process set forth in claim 1 in which the hydrosulfite concentration is not less than 0.25% and the temperature is from C. to 105 C.

10. A process of manufacturing paper pulp which comprises the steps of pre-cooking colored rags under a pressure of 15 to 70 lbs. per square inch with a mixture of lime and soda ash at a concentration of from 3% to 6% lime and 1% to 2% soda ash, washing with water, removing the dyestuif by cooking with not less than 0.25% of sodium hydrosulfite at a temperature between 80 C. to 105 C., removing the residual hydrosulfite liquor from the rags, and again washing the rags with water.

11. A process of manufacturing paper pulp which comprises the step of pre-cooking colored rags under a pressure of 1'7 to '70 lbs. per square inch with a concentration of 2% to 6% caustic soda, washing with water, removing the dyestuff by cooking with not less than 0.25% of sodium hydrosulfite at a temperature between 80 C. to 105 C., removing the residual hydrosulfite liquor from the rags, and again washing the rags with water.

SAMUEL LENHER. HARRY FLETCHER. LEWIS. 

